Bug 1500064 - Use GCHashSet in property enumeration code. r=jonco
This changes Maybe<IdSet> to Rooted<IdSet>. I think the Maybe<>
is not buying us much because HashTable's storage is now lazily
allocated (bug 1481998). I didn't notice any regressions in
micro-benchmarks; this patch might be a small improvement actually.
Differential Revision: https://phabricator.services.mozilla.com/D9225